As an example of an illumination module, an illumination module for backlight using light emitting diodes (LEDs) is used as a large emitting-surface type illumination system such as a backlight unit for a TV set. Illumination modules for backlight using LEDs are divided into two types. One is an edge-light type in which lateral-emitting LEDs are arranged on the periphery of a backlight unit and lights from the LEDs are guided by a light-guiding plate to realize surface illuminant. The other is a direct type in which many LEDs which emit lights in the upward direction are arranged on a plane to realize surface illuminant. The edge-light type is advantageous to slimming down. On the other hand, the direct type is advantageous to improvement of image quality and energy-saving, because amount of lights to be emitted by each LED can be controlled depending on contrast of an image to be displayed.
FIG. 1 shows a plan view of an illumination module of direct type for backlight. The illumination module for backlight of illumination module includes an enclosure 209, a reflecting plate 203, boards 207 and illumination lenses 120. The reflecting plate 203 is provided at the bottom of the enclosure 209 and a number of elongated boards 207 are arranged in parallel to one another on the reflecting plate 203. On each of the boards 207, illumination lenses 120 including LEDs are arranged at regular intervals. As a result, illumination lenses 120 including LEDs are arranged at positions of lattice points of a rectangular lattice as shown in FIG. 1.
FIG. 2 shows a side view of the illumination module of direct type for backlight. Lights emitted from an LED 201 reach a diffusion plate 205 thorough the illumination lens 120. A portion of lights which have reached the diffusion plate 205 pass thorough and exit from the diffusion plate 205. The other portion of lights which have reached the diffusion plate 205 are reflected on the diffusion plate 205, reach the reflecting plate 203, are reflected on the reflecting plate 203, again reach the diffusion plate 205, pass thorough and exit from the diffusion plate 205. Thus, the illumination lenses 120, the reflecting plate 203, and the diffusion plate 205 are used such that brightness of lights emitted from the diffusion plate 205 is made even all over the diffusion plate 205.
FIG. 3 shows a side view of an illumination module of another direct type for backlight. Lights emitted from an LED 201 are reflected on a surface of an illumination lens, reach the reflecting plate 203, are reflected on the reflecting plate 203, reach the diffusion plate 205, pass thorough and exit from the diffusion plate 205. With such a construction, an illumination module of direct type for backlight, in which slimness and evenness of brightness are further enhanced, can be realized.
However, in conventional illumination modules of direct type for backlight, brightness of a diffusion plate has not been made even to a sufficient degree.
FIG. 4 illustrates unevenness of brightness of a diffusion plate of conventional illumination modules of direct type for backlight. In FIG. 4, illumination lenses 120 including LEDs 201 are arranged at positions of lattice points of a rectangular lattice. In FIG. 4, an area illuminated by a single illumination lens 120 including an LED 201 is marked with IL. The shape of the illumination lens 120 is symmetric about an axis, and therefore the illuminated area is circular. On the other hand, the illumination lenses 120 are arranged at positions of lattice points of the rectangular lattice as described above, and therefore a bright area R1 with a higher brightness and a dark area R2 with a lower brightness are generated on the diffusion plate.
In order to eliminate above-described unevenness of brightness, illumination lenses which are rectangularly shaped in outline have been developed (Patent Documents 1 and 2). However, in an illumination lens rectangularly shaped in outline, incident angles of rays which come from the light source and exit from the lens, are larger than those in a lens of axial symmetry. Accordingly, components of Fresnel reflection and total reflection increase, thus generating unevenness of illuminance on a plane to be illuminated. In order to prevent such unevenness of illuminance, the lens has to be upsized.
As another way to eliminate unevenness of brightness, a layout in which light sources are arranged in hexagonal lattice is proposed (Patent Documents 3 and 4). However, when this layout is employed, an area to be illuminated is also formed based on hexagonal lattice, and therefore there exists a mismatch between the area and an elongated rectangular range to be illuminated, which is generally required for backlight of TV sets or the like, along the outer edge. Accordingly, there arises a problem that illumination lights cannot effectively be used.
Thus, an illumination lens which makes brightness of an illumination module which is constructed by light sources arranged in rectangular lattice even and an illumination module of even brightness, which is constructed by light sources arranged in rectangular lattice have not been developed.